The rapid growth of automatic measurement techniques for precision products, ranging from mechanical parts made to very narrow tolerances to minute VLSI semiconductor products, has led to the generation of a number of systems for automatic inspection of such parts and components. In these systems, an article to be checked or measured is imaged, generally through a high magnification optical system, on an electronic image pickup device, such as a Vidicon, CCD array, or other image-to-signal converter. The video signals can be processed, utilizing adapted software and a microprocessor or minicomputer, to analyze the image and particular parts of the image. Image transformation techniques can be used to rotate, zoom and translate the image, pattern recognition techniques can be employed to compare the image to a predetermined standard, flaws can be identified, dimensions and spacings can be measured. These functions can be performed with high reliability and freedom from error when the image is adequately defined.
The signals derived from scanning of the optical image, however, depend on the physical and optical characteristics of the object under examination. While the eye can readily perceive differences in color and texture, and the eye can also adjust for reflections and other effects and a human operator can make adjustments dependent on overall perceptions, processing of the image signal depends essentially upon the nature of the variations in the signal. Precise location of an edge of a given surface, for example, requires a high signal contrast between the surface and its background. If the diameter of an aperture is being measured, for example, abrupt variations in signal magnitude enable the vision system to give a reading of the dimension that is much more rapid, accurate and reliable than any system requiring operator judgment. Precision components, however, are three-dimensional in character and it is often desirable to inspect or measure specific attributes at different levels. If a hole is tapered, for example, and the hole is concentric with a vertical axis, the taper can be determined with accuracy by taking hole diameter readings at each end, for which purpose uniform illumination is not optimal. This illustrates the general problem of so illuminating an object under inspection that highlights and shadows can be used to best advantage in displaying the features and surfaces that are to be inspected or measured.
The vision systems of the class described herein are best exemplified by the View Engineering Model 1200, a microprocessor-based system which makes non-contact measurements in three dimensions of complex parts and to an accuracy of the order of 0.00025". In this system, object illumination may be provided by a light source disposed around the objective lens of the magnification system for the camera, or coaxially through the objective. The capability for enhancing different characteristics at different times has been limited except that limited advantages have been derived by using discretely differing light sources in particular instances.
An illumination system which can efficiently and economically provide different, controllable, illumination of an object under study is not limited to use with vision systems of the type generally described. It can also be employed in microscopy, microphotometry, and microphotography, where the part being examined is viewed under some substantial magnification and image enhancement is desirable for specific purposes.